Exemplary electrophotographic image forming apparatuses include an optical scanning apparatus. In the optical scanning apparatus, light emitted from a light source is guided to a photosensitive member, whose surface is to be scanned, by a light deflector (a polygon mirror, for example), whereby a latent image is formed.
PTL 1 discloses an image forming apparatus including an array of surface emitting lasers (vertical-cavity surface emitting lasers, abbreviated to VCSELs) as a light source. It is easy to provide a two-dimensional array of VCSELs.
In the above image forming apparatus, a laser beam emitted from the array of surface emitting lasers is collimated into substantially parallel rays by a collimator lens and is then guided to a deflecting surface of a rotating polygon mirror, i.e., a light deflector. The beam deflected by the polygon mirror travels through an imaging optical system (an fθ lens system) and is focused in the form of a spot on a scanning surface. The scanning surface is scanned at a constant speed with the focused beam.
In the above image forming apparatus, the collimated beam emerging from the collimator lens in a sub-scanning direction (contained in a sub-scanning section), which is orthogonal to the deflecting direction (a main scanning direction), is focused on or near the deflecting surface by a cylindrical lens. Subsequently, the beam is refocused on the scanning surface by the imaging optical system. That is, the image forming apparatus employs a tilt correction optical system.
FIG. 21 is a schematic sectional view of a surface emitting laser 1400.
In the surface emitting laser 1400, a plurality of semiconductor layers including a rear mirror 112, an active layer 114, and a front mirror 116 are provided on a semiconductor substrate 110, whereby a vertical cavity is formed. The active layer 114 and the front mirror 116 are partially etched, thereby forming a mesa structure.
A current confinement structure 118 is provided in the front mirror 116. The current confinement structure 118 regulates the electric current flowing through the active layer 114 and defines an emission area of the active layer 114. The current confinement structure 118 is formed by, for example, oxidizing a semiconductor layer made of AlGaAs or the like from the sidewall of the mesa structure.
The current confinement structure 118 formed by oxidizing such a semiconductor layer has a lower refractive index in an insulator portion thereof than in a semiconductor portion thereof. That is, the current confinement structure 118 has a higher refractive index in a central portion thereof than in a peripheral portion thereof. Such a structure is referred to as a waveguide structure. Thus, the current confinement structure 118 defines the profiles of resonance modes of the cavity including a fundamental mode 130.
When an electric current is supplied to the active layer 114 between a rear electrode 120 provided on the rear surface of the substrate 110 and a front electrode 122 provided on the front surface of the front mirror 116, the cavity formed by the combination of the front mirror 116 and the rear mirror 112 causes the surface emitting laser 1400 to oscillate. A protective film 124 made of a dielectric material or the like is provided on the output surface, i.e., the front surface of the front mirror 116.
In a general image forming apparatus including an optical scanning system, a latent image represented by a single-peak pattern is formed on a scanning surface. Therefore, the oscillation mode (the transverse mode) of a surface emitting laser to be utilized is in general the fundamental mode (the lowest-order mode).
As illustrated in FIG. 21, the fundamental mode 130 has a single-peak intensity distribution in the cavity. In general, the profile of the electric field amplitude of the fundamental mode 130 can be approximated to a Gaussian function. That is, a beam emitted from a surface emitting laser that oscillates in the fundamental mode that is a single transverse mode is usually a Gaussian beam.
Here, the complex amplitude (amplitude and phase) of an electric field in a plane immediately after the output surface is referred to as the near field complex amplitude. The amplitude, intensity, and phase of the electric field are referred to as the near field amplitude, the near field intensity, and the near field phase, respectively. The distribution of near field intensity is referred to as the near field pattern (NFP).
Furthermore, the complex amplitude (amplitude and phase) of an electric field in a spherical plane defined by a radius ∞ and centered on the light source is referred to as the far field complex amplitude. The amplitude, intensity, and phase of the electric field are referred to as the far field amplitude, the far field intensity, and the far field phase, respectively. The distribution of far field intensity is referred to as the far field pattern (FFP).
FIG. 22A is a set of graphs representing an exemplary near field complex amplitude (amplitude and phase) of the fundamental mode 130 of the surface emitting laser 1400. FIG. 22B is a set of graphs representing an exemplary far field complex amplitude (amplitude and phase) of the fundamental mode 130 of the surface emitting laser 1400.
A profile 132 representing the near field complex amplitude of the fundamental mode 130 of the surface emitting laser 1400 illustrated in FIG. 21 has a Gaussian shape that is substantially the same as the profile of the fundamental mode 130 in the cavity. According to the Fraunhofer diffraction theory, the near field complex amplitude and the far field complex amplitude are the Fourier transformations of each other. Therefore, if the NFP of a fundamental mode is substantially Gaussian, the FFP of that fundamental mode is also substantially Gaussian. As illustrated in FIGS. 22A and 22B, the phases of the near field complex amplitude and the far field complex amplitude are constant at 0 in this case.